The Mirror Drum
A very early improvement over the Nipkow disk was the Mirror drum. For its
size, it was a most light efficient mechanical scanner. In its original form,
there were as many mirrors as there were lines in the picture and each mirror was tilted at a different
angle compared to the axis of the drum. For horizontal scanning, the mirror drum
axle was supported in a vertical plane. Therefore for vertical scanning, the
axle was horizontal. As it rotated, each mirror caused a line to be scanned
below (horizontal scan) or beside (vertical scan) the previous one. The drawing
shown below is of a basic vertically scanned mirror drum system.
In this drawing, the modulated light originates at a small
aperture at the lower left, then passes through a projection lens. This lens is
adjusted so that the the light at the aperture itself is focused on the screen.
Of course, in order for the light to get there, it must be reflected off of
mirrors on the drum. The mirrors are thereby able to control the position of the
light spot on the screen as it rotates. Since each mirror is carefully set at a
different but proper angle, as the drum rotates, the focused light spot takes on
the appearance of a set of parallel horizontal or vertical lines, commonly
referred to as a raster. With proper modulation of the light, you have a
picture. |
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The light source used in large screen theater systems might have been a
modulated arc or a standard arc lamp, the latter requiring a Kerr cell along
with its associated components. In later years, the arc light was replaced with
powerful incandescent lamps. For receivers used in the home, crater arc lamps
were able to provide sufficient image brightness.
This is a picture of a mirror drum receiver manufactured by John Logie Baird
in 1932. It provided a 30 line image of 9 inches by 4 inches. A larger version
of this receiver produced a 14 inch by 6 inch picture. This receiver used a 100 watt incandescent lamp as a white light source, that was in turn
modulated with the video signal by a Baird "Grid Cell". This was Baird's version
of the Kerr cell.
Beyond 60 lines with mirrors of a reasonable size, the drum became tended to
be large. The mirrors were also difficult to adjust. Shown in the two photos
below are three examples of recently made 32 line mirror drums. The mirrors are
of the very efficient first surface variety and measure approximately one inch
square. The drum itself is nine inches in diameter. Each mirror has three adjustment
screws. The drum is driven by its shaft through a tension spring in order to
reduce any tendency for the drum to "hunt".
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There was constant pressure to increase the number of lines in the image.
However, this tended to increase the size of the scanning assembly as well the
signal bandwidth requirements. Commercial television started with 24 line
pictures, mostly because of the bandwidth limitations of transmitters operating
in or just above the Broadcast Band. As the transmissions were allowed to move
up in frequency, more lines could be added to the image, thereby improving image
resolution. With pictures up to 60 lines, scanning disks/drums with holes,
lenses or mirrors could provide acceptable pictures with disks of reasonable
size. Beyond that, ( 120 lines and up) the holes, lenses or mirrors became too small or the
disk/drum too large and the requirement for manufacturing precision too great
for scanning disks to see further use in receivers. By 1932, the Nipkow disk and
mirror drum had both pretty much had their day. This was also the end of the
"low definition" era of television. With 60 lines (or less), television images
were limited to close-ups of actors or scenes with large recognizable
objects.
The Mihaly-Traub
scanner overcame the disadvantages of the mirror drums in a very clever way.
From this time on, there was a major effort to develop small, low cost scanners
that would produce large, bright pictures, with improved resolution. The scanner
shown here on the right is an example of a drum able to produce 120 line images.
At 120 lines and more, television takes on movie like qualities, with "long
shots" as well as close-ups, where the scenery added to the impact of the
actors. This was the beginning of high definition television (HDTV), and some
very innovative ideas were tried to make it happen. Continuing efforts to
develop small light efficient mechanical scanning assemblies, tended to fall
into two general categories; vibratory scanners and rotating scanners. |
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